1. Field of the Invention
The present invention relates to a transducing element of a stack (laminate) type for transducing electric energy to mechanical energy and also relates to a vibration drive device using the transducing element.
2. Related Background Art
For example, in electromechanical transducing materials of piezoelectric ceramics or the like having the electromechanical transducing capability, there is a conventionally proposed stack (laminate) element with alternate lamination of electrode material and piezoelectric ceramic.
When compared, for example, with a single piezoelectric ceramic plate having the same thickness, this stack (laminate) element can achieve greater deformation strain and greater generating force with a lower voltage applied thereto and, therefore, it is utilized or investigated in recent years, particularly as a drive section of vibrator forming a vibration drive device such as a piezoelectric actuator or a vibration wave motor.
There are typically two methods for fabricating this stack (laminate) element as described below.
The first fabrication method is a method for providing electrode layers on the both surfaces of each baked piezoelectric ceramic ply, stacking (laminating) a plurality of such plies, and bonding them, for example, with an adhesive or the like.
The second fabrication method is a sinter integration method for stacking (laminating) layers of molded sheets (green sheets) containing an organic binder of piezoelectric ceramic before sintering and layers of electrode paste before sintering, thermally pressing then into an incorporated form, and sintering it.
The second fabrication method is drawing attention these years, and can fabricate a compact and higher-performance stack (laminate) element, because the piezoelectric ceramic layers can be formed thinner and because the thermal press can obviate a need for use of the material such as the adhesive. However, the fabrication processes thereof become technically more complex.
The stack (laminate) element will be described referring to FIGS. 10A, 10B and FIG. 11. FIG. 10A is a top plan view of the stack (laminate) element, FIG. 10B an example of transverse cross section, and FIG. 11 a longitudinal cross-sectional view of the stack (laminate) element 11 shown in FIG. 10A and FIG. 10B. A plurality of piezoelectric ceramics 1 forming the stack (laminate) element 11 are formed in a disk shape having the thickness of about 100 .mu.m and having an inner diameter.
On the piezoelectric ceramics 1 an electrode layer 2 or 3 is formed over the almost the entire region of each one surface and an uppermost electrode layer 7 over the uppermost surface. These electrode layers 2, 3, 7 are formed by screen printing of a paste of electrode material over each one surface of piezoelectric ceramic 1.
Another electrode layer 8 is formed over a surface opposite to the electrode layer 3 of the piezoelectric ceramic located in the lowermost of the stacked (laminated) layers. Thicknesses of these electrode layers are usually approximately several .mu.m or less.
After the electrode layers are formed on each of the plural piezoelectric ceramics 1 before sintering, the plural piezoelectric ceramics 1 are stacked (laminated) as aligned at predetermined positions, are thermally pressed, and are sintered integrally, thereby making the stack (laminate) element.
After that, two side electrodes 9, 10 are formed, for example, by baking of silver paste on stacked (laminated) side portions of the stack (laminate) element. Further, lead wires not illustrated are connected each to the side electrodes 9, 10 to enable electric conduction with the outside.
Further, an electric insulation process is conducted with a resin material or the like for achieving electric insulation between the edges of the electrode layers and the outside, thus completing the stack (laminate) element 11.
Here, each of the electrode layers 2 is formed so as to cover almost the entire one surface of piezoelectric ceramic 1. The electrode layers 2 are made electrically conductive with the side electrode 9 at the side surfaces of piezoelectric ceramics 1. The electrode layers 2 are not formed around the side electrode 10 in order to maintain electric insulation to the side electrode 10.
The electrode layer 8 formed on the lowermost layer of piezoelectric ceramic 1 is formed in the same shape as the electrode layers 2 so as to be electrically conductive with the side electrode 9, but so as to be electrically insulated from the side electrode 10.
Similarly, the electrode layers 3 are formed almost over the entire surfaces of piezoelectric ceramics 1 except for the surroundings of the side electrode 9 so as to be electrically conductive with the side electrode 10, but so as to be electrically insulated from the side electrode 9. The electrode layer 7 formed on the uppermost layer of piezoelectric ceramic 1 is also formed so as to be electrically conductive with the side electrode 10, but so as to be electrically insulated from the side electrode 9. Then a polarization process is carried out after the integral sintering.
The arrows in FIG. 11 indicate directions of polarization in the stack (laminate) element. When the side electrode 9 is kept at a positive potential relative to the side electrode 10, the stack (laminate) element is polarized as indicated by the arrows in the draws. When an electric field to make the side electrode 9 positive relative to the side electrode 10 is applied to the stack (laminate) element thus polarized, the stack (laminate) piezoelectric element 11 is deformed to expand in the stack (laminate) direction; when a reverse electric field is applied thereto, the stack (laminate) element is deformed to contract. When an alternating electric field is applied as the electric field, expansion and contraction is repeated so as to induce vibration.
FIG. 12 is a longitudinal cross-sectional view of a bolted Langevin type vibration drive device using the stack (laminate) element 11, which is an example of the vibration drive device. Electrode plates 13 formed of copper plates or the like are placed on both end faces of the stack (laminate) element 11 and vibrating bodies 15 made of a metal material are disposed so as to pinch these stack (laminate) element 11 and electrode plates 13 from both sides.
An insulating sheet 14 made of a resin material is placed between one vibrating body 15 and associated electrode plate 13. These two vibrating bodies 15 and a fasting bolt 12 of a metal material are screwed with each other, thus forming the vibration drive device 20.
When an alternating electric field having a frequency close to the natural frequency in an expansion/contraction mode in the axial direction of the device or close to the resonance frequency in the proximity thereof is placed between the two electrode plates 13, the vibration drive device 20 is driven in a state near the state of resonance, whereby it can generate vibrational energy.
Now, describing the electrode layers formed between the layers of the sintering integration type stack (laminate) element, the electrode layers are formed by applying the electrode paste in a predetermined shape onto the piezoelectric ceramic green sheet of each layer of the stack (laminate) element by the screen printing method to be integrated with the green sheet and baking the integral body to sinter the electrode layers together with the green sheets.
The electrode paste is made by mixing metal particles becoming the electrode material, with an organic binder, a solvent, a filler, and an inorganic additive such as glass. The piezoelectric ceramics usually contain a main ingredient of lead zirconate titanate, and are sintered in the air and lead atmosphere at 1000 to 1300.degree. C. Therefore, the electrode material is selected from very limited materials such as noble metals, for example gold, platinum, palladium, and silver, which are neither melted nor oxidized at the baking temperatures. However, taking the material cost into consideration, it was common to mainly use cheap silver among them.
In applications of the foregoing stack (laminate) element to various vibration applying devices, there are roughly the following two ways of application.
One of them is a way for applying a dc voltage to the stack (laminate) element and statically utilizing displacement thereof or force generated.
The other one is a way for applying an alternating voltage to the device to vibrate the vibration system including the stack (laminate) element and utilizing vibration thereof while keeping the system in a state close to the state of resonance with large amplitude of vibration.
There are many examples of the former relatively well known, but there are little examples of the latter to date. An example of the latter is the bolted Langevin type vibration drive device incorporating the above-stated stack (laminate) element, and another example is a vibration wave motor recently developed.
The present inventors made the stack (laminate) element that was able to be used in the state of resonance and have conducted various investigations using the stack (laminate) element in the vibration wave motor. It was then found that when the vibrating members of the vibration wave motor were vibrated by the stack (laminate) element to be driven in a state close to the state of resonance, the stack (laminate) element using the electrode material containing a large amount of cheap silver as before showed great losses of vibrational energy upon vibration to increase generation of heat, which degraded the performance, is especially efficiency, of the motor.
It is the present status that the stack (laminate) element used in the state of resonance normally needs to be made of a piezoelectric ceramic material with little damping, i.e., of a material with high Q value of so-called mechanical quality factor and that there is no such stack (laminate) element commercially available at present.
Especially, in the stack (laminate) element used for the vibration wave motor, there is out-of-plane bending and torsional deformation in the stack (laminate) surfaces, which causes shear strain in the electrode layers. Therefore, damping of vibrational energy becomes especially extreme.